There is well known a hydraulic control apparatus for a vehicular automatic transmission, which is configured to switch a lock-up clutch provided in a fluid-operated power transmitting device, between its lock-up on (engaged) state and its lock-up off (released) state. Patent Document 1 discloses an example of such a hydraulic control apparatus for a vehicular transmission. The hydraulic control apparatus disclosed in Patent Document 1 is provided with a first pressure regulating valve of a pressure-relief type for regulating a pressure of a working oil delivered from an oil pump, to a first line pressure, and a second pressure regulating valve for regulating the first line pressure of the working oil discharged from the first pressure regulating valve, to a second line pressure, and is configured to switch the lock up clutch between its lock-up on and off states, with a lock-up switching valve configured to select respective passages of supply of the working oil of the second line pressure to the fluid-operated power transmitting device.
An increase of an amount of flow of the working oil of the first line pressure into a primary system may cause a decrease of an amount of the working oil discharged from the first pressure regulating valve, and an insufficient amount of flow of the working oil of the second line pressure into a secondary system, leading to a failure to hold the lock-up clutch in the lock-up on state. To avoid this problem, a hydraulic control circuit 1 as shown in FIG. 7 for example is proposed. In this hydraulic control circuit 1, the pressure of the working oil delivered from an oil pump 2 is regulated to a first line pressure PL1 by a first pressure regulating valve 3 of a pressure-relief type, and the working oil of the first line pressure PL1 flows into a primary system LPL1, while the pressure of the working oil discharged from the first pressure regulating valve 3 is regulated to a second line pressure PL2 by a second pressure regulating valve 5 of a pressure-relief type, and the working oil of the second line pressure PL2 flows into a secondary system LPL2. The hydraulic control circuit 1 is also arranged such that the working oil is kept supplied from the primary system LPL1 to the secondary system LPL2 via an orifice 4. Namely, the proposed hydraulic control circuit 1 is configured such that the first line pressure PL1 is regulated to a constant modulator pressure PM by a modulator valve 6, so that the working oil of the modulator pressure PM is supplied to a lock-up clutch 9 in a torque converter 8 via the orifice 4 and a lock-up switching valve 7, in addition to the working oil of the second line pressure PL2, for permitting the lock-up clutch 9 to be held in the lock-up on state.